1. Field of the Invention
The invention relates to a cellular mobile radio system comprising fixed stations and mobile stations, in which system a radio channel in a radio cell is allocated via a channel list. The invention likewise relates to a controller for a fixed station of such a mobile radio system.
2. Description of the Related Art
Mobile radio systems for covering rather large coverage areas have been
structured as cellular radio networks for a rather long time. Each cell of such a radio network comprises at least one fixed station which provides the radio connection to the mobile stations located in its radio cell. A special importance is attached to radio network planning in order to make the available frequency band for the radio system accessible to a maximum number of users in the whole system.
In the radio network planning each fixed station is allocated a limited selection of radio channels from the total number of available radio channels. In adjacent radio cells mutually different interference of channels are used, so that radio channels with each other can be avoided. Due to the limited width of the frequency band, however, the radio channels in the whole radio network must be reused as often as possible to obtain maximum user capacity. The ratio of the total number of radio cells to the number of times the same set of frequencies are re-used is denoted here as the re-use factor.
When a channel needs allocation plan is made, also the channel need of each radio cell must be considered. The channel need of a radio cell is determined by the size of the radio cell and the radio traffic to be expected in that cell. For example, more radio channels are needed in the coverage area of a large city than in an entirely rural area. Partly for this reason, there are already radio cells of variable size. In so-called small cells, which are accordingly of reduced transmitter power, it is possible to re-use a radio channel at a rather small distance from the small cell.
The distance these radio cells are to be kept apart to avoid radio interference depends on many factors. These factors are, for example, the transmitter power and the conditions concerning radio wave propagation. The conditions concerning radio wave propagation are especially the result of shadows, bends, scatterings, reflections and multipath propagation on the radio transmission path.
When radio cells are planned, topographical data (such as differences in height, buildings and so on) are modelled by statistical methods, or the data are collected by measurements taken during rides with instrument cars to allow for requirements concerning radio wave propagation. By implementing methods such as, for example, graph colouring, mutually interfering radio channels can be allocated to radio cells which are sufficiently far apart. In this manner a specific number of radio channels is fixedly allocated to each radio cell as a channel list from the beginning. If there is a request for a channel for communication with a particular mobile station, an arbitrary free channel is selected from the allocated channel list of such cell.
In contrast, with the so-termed dynamic channel allocation methods, it is tested only when a channel is to be selected whether such channel is compatible with the channel seizure occurring in the network at that moment. If it is not compatible, a following channel is selected. Since the channel compatibility is to be tested for each individual case, a dynamic channel allocation method can extend the channel list allocated to each radio cell to all the available channels.
EP 0 202 485 A2 has disclosed a dynamic channel allocation method which is also known by the name of channel segregation. With this method each channel is additionally assigned a priority. When there is a request for a channel, the channels are searched for their compatibility with a descending priority. If a channel is available its priority is increased; if a channel is not available, its priority is reduced. By giving priority to the channels, there is achieved that the order in which the channels in the individual radio cells are searched is constantly changed, while in each radio cell the channels having a high priority are given preference. By increasing or reducing the priorities, there is a possibility for the channel allocation in the radio network to be optimized or reorganized.
When there is a limited number of channels available, the increase of the number of radio channels allocated to a particular radio cell is always at the cost of the available channels in the adjacent radio cells. Due to the enhancing need for mobile communication, both the already existing mobile radio networks and those being laid out will in the near future reach their capacity limits.
Only by the simplifications realised on the basis of statistics can the planning according to known methods be kept within justifiable limits. But especially with small cells the statistical simplifications are no longer tolerable, because the calculated data become increasingly unreliable. Minor changes of the infrastructure of a small radio cell, even if they are of a momentary nature, may lead to a completely different situation from the point of view of radio engineering or traffic load, so that regular operation of the radio cell cannot be maintained with the preplanned data.
For example, road construction causing a constant traffic backup also causes the number of participants in the (radio) traffic in a cell to increase drastically. Since it is impossible to have a sufficient number of channels available in each radio cell for any conceivable situation, due to the limited number of available channels, modified radio and (radio) traffic situations lead to overloaded radio cells and the attendant disadvantages, such as rejecting a request for a call or terminating a call due to the lack of a free channel.